A massive summer mayfly swarm, a grotesque natural phenomenon calling to mind a biblical plague or an Alfred Hitchcock film, swept across the Mississippi River Valley late Sunday from Red Wing, Minn., to Prairie du Chien, Wis.

Billions of the short-lived bugs began emerging to hatch just after 8:30 p.m. — so thick that they appeared on radar as a minor rain squall — and then peaked about 9:45 p.m., said Zack Taylor, a meteorologist with the National Weather Service in La Crosse, Wis.

The onslaught subsided several hours later, but not before leaving behind a huge, slimy mess and getting the blame for a three-vehicle accident on the Hwy. 63 bridge linking Red Wing and Hager City, Wis., that left one person hospitalized.

Mayflies swarm a few times each summer to mate and hatch, Taylor said.

"What made this unique was the massive number of insects that were involved," he said. "The signature on the radar was pretty impressive."

This was the first swarm of 2014. Last year along the Mississippi, similar swarms happened on June 15, 18 and 25, and on July 14.

Two summers ago, another immense mayfly swarm caused collisions and brought out snowplows for rare summer cleanup duty, and forced the lights to be doused on the Wakota Bridge on Interstate 494 near South St. Paul to keep the bugs away.

Sunday's accident happened about 10:25 p.m., when a northbound car driven by Theresa L. Hunt, 24, of Ellsworth, Wis., lost control on the slickened roadway as the blizzard-like flurry of insects cut visibility, according to the Pierce County Sheriff's Office.

Hunt's car hit another vehicle heading south driven by Scott Pauly, 52, of Rochester, and then a second vehicle, a van driven by Tanya Lapointe, 35, of Baldwin, Wis.

Hunt was slightly injured, and Cynthia Pauly, 51, a passenger in one of the struck vehicles, was taken to Mayo Hospital in Red Wing for treatment of undetermined injuries.

For all the problems they cause, mayflies and their activity actually signify good water quality, according to the Minnesota Department of Natural Resources (DNR).

A cornerstone of the aquatic food web, the mayfly is very sensitive to chemical pollutants, increases in sediment and reductions in dissolved oxygen levels in the water — mainly from organic sludge.

By the 1980s, mayflies, stoneflies, caddis flies and even some species of fish had disappeared from the Mississippi River from the Twin Cities area all the way to Lake Pepin.

But modern sewage treatment facilities and regulations on disposal of toxic chemicals have brought mayflies, and a healthy food chain, back to most waterways.

Mayflies spend about 99 percent of their lives as nymphs in rivers and other freshwater bodies, according to the DNR. Sometimes they emerge in small batches, but once in a while — in one of those peculiarities of nature — they burst forth en masse in a frenzied mating party.

Mating takes place in flight as females fly through swarms of males, who grab them with their long front legs. Females return to the water to lay eggs and then die, providing an easy feast for fish but frustrating anglers.

The males don't fare much better.

They die about a month later, if not sooner, although the mission of perpetuating the species is completed with the eggs falling to the bottom of the water to start the cycle again.

Jim Anderson • 651-925-5039 Twitter: @StribJAnderson